The present invention relates to machine tools and, more particularly, to machine tools for cutting screw threads in or upon a workpiece.
In the low-volume production of machined parts, thread cutting is oftentimes preformed manually or with the aid of an operator-controlled machine. In the latter situation, for example, a thread cutting tap is mounted in a friction-drive tapping collet, or similar device, which is carried by the rotating spindle of a drill press or similar user-operated machine. The rotating spindle and the tap-carrying collet are advanced by the machine operator toward and into the to-be-threaded bore and retracted after the thread cutting operation is completed. The use of an operator-controlled machine is oftentimes preferred in low-volume production contexts because the cost of each tapping cycle and overall productivity may not be paramount considerations.
Where thread cutting is required at minimum threading cycle costs in high volume production situations, specialized thread tapping work stations have been developed for use as a component in a larger, automated machine system. One such thread cutting work station, known as a leadscrew tapper, includes a rotatably mounted spindle that carries a tap-holding chuck or collet at one end. The spindle is externally threaded at a thread pitch equal to that of the threads to be cut and is also carried in an internally threaded, stationary sleeve so that rotation of the spindle in one direction or the other causes it to advance or retract axially. A hydraulic or other type motor is coupled to the spindle and rotates the spindle in a first direction to advance the spindle and tap toward and into the workpiece to cut the desired threads. After completion of the thread cutting step, the hydraulic motor is reversed to drive the spindle in the opposite direction to retract the tap from the now-threaded bore and return the tap to its initial, home position.
Work stations of the type described function well although the requirement for an externally threaded spindle and matching internally threaded collar for each of the standard thread pitches increases initial fabrication costs of the work station as well as the changeover time for changing from one thread pitch to another. Additionally, the use of screw threads to advance or retract the spindle has attendant operational disadvantages. For example, where a hydraulic motor is used to drive the spindle, the motor is typically driven at full line pressure throughout the entire tapping cycle, that is, when the rotating spindle is advanced under no-load conditions toward the workpiece, during thread cutting, and when the counter rotated spindle is retracted under no-load conditions toward its home position. Thus, the tapping of a few threads in a relatively thin wall section of a workpiece can require a relatively large volume of high pressure hydraulic fluid for full-power driving of the hydraulic motor during a tapping cycle in which thread cutting is effected for a relatively small fraction of the total cycle. Since the externally threaded spindle is advanced or retracted along its operating axis at a feed rate controlled, in part, by the thread pitch of the externally threaded spindle and its supporting collar, the feed rate of the spindle to and from the workpiece is typically fixed by the maximum feed rate for the thread cutting portion of the tapping cycle. Where the distance between the to-be-threaded bore and the home position of the tap is not large, this feed rate limitation does not materially affect the time duration of the tapping cycle or the overall production rate. On the other hand, where the distance between the to-be-threaded bore and the initial position of the tap is large, the time required for the rotating spindle to traverse the distance to the workpiece, effect thread cutting, and retract to its home position can be sufficiently long to adversely lengthen the tapping cycle and affect the overall production rate and associated costs.
In another type of thread cutting work station, a rotary drive motor is mounted on a stationary support and coupled through a splined connection to a rotatably mounted, tap-carrying quill that can be advanced and retracted relative its drive motor to and from a to-be-threaded bore. In this type of arrangement, the axial extension of the splined connection must be such to accommodate the distance between the initial or home position of the tap and the to-be-threaded bore, and, accordingly, this type of arrangement does not allow for the convenient fabrication of compact, modular thread cutting work stations.